1. Field of the Invention
The present invention relates to a hydraulic control circuit in a hydraulic excavator. More particularly, the invention is concerned with a system for changing over the responsivity of an attachment attached to the front portion of a hydraulic excavator.
2. Description of the Prior Art
FIG. 6 is a hydraulic circuit diagram described as FIG. 1 in Japanese Patent Laid Open No. 207697/95. A hydraulic excavator shown in FIG. 6 is equipped at the front portion thereof with a working attachment 1 comprising a boom 2, an arm 3 and a bucket 25. When time t0 required for actually depressing a lever is shorter than time T which has been preset for the lever depressing operation (time set by a lever depression time setting device 30), if a lever 15 of a remote control valve 12 for boom is operated, a pilot pressure acts shorter than the preset time T on a boom lowering pilot port 22 of a pilot change-over valve 8 for boom. The pilot pressure is detected by a pressure sensor 28 and the detected pressure signal is fed to a controller 29. On the basis of this pressure signal the controller 29 judges that the actual lever depressing time t0 is shorter than the preset time T, and does not output a throttled oil path position change-over command signal to an electromagnetic change-over valve 26. Since a solenoid 27 is deenergized and the electromagnetic change-over valve 26 is in an open oil path position ".alpha.", it is possible to perform operation in high responsivity. That is, it is possible to conduct a bucket striking work which requires a quick operation.
In the prior art shown in FIG. 6, however, when a work involving a quick operation which requires a high operation responsivity (for example, a ground hardening work with the back of a bucket, a soil scattering work, or a mud removing work from the machine body) is to be performed, it is necessary that a repeated change-over operation be done for the lever 15 of the remote control valve 12 for boom in a shorter time than the preset time T. This limitation imposed on the operation time has been inconvenient in executing operations.
In the prior art shown in FIG. 6, moreover, no means have been taken for effecting slow works which require a low operation responsivity (for example, a slope face correcting work, a leveling work, and a load lifting work).